This invention relates generally to gradient field coil systems for image-generating apparatus which is used in nuclear magnetic resonance tomography, and more particularly, to an arrangement where a gradient coil system is arranged on at least one hollow cylindrical support body wherein a magnetic field has a field gradient which is essentially constant in the imaging region.
In image-generating apparatus for nuclear magnetic resonance technology, particularly for zeugmatography, a gradient coil field system is arranged on at least one hollow cylindrical support body having a radius r; the cylinder having an axis which extends in the z-direction of an orthogonal x, y, and z coordinate system with the coordinate origin in the center of the imaging region. In this system, a base field magnet is oriented to produce a magnetic field B.sub.z in the z-direction. Magnetic field B.sub.z is formed by at least two ring-shaped individual coils which are arranged approximately symmetrically with respect to the x-y plane through the center of the imaging region to produce a field gradient G.sub.z =.delta.B.sub.z /.delta.z. Current flows in opposite directions through the two ring-shaped coils. The coil system further contains at least one set of a pair of saddle-shaped individual coils which are arranged at least approximately symmetrically with respect to the plane of symmetry, and which are provided for generating field gradients G.sub.x =.delta.B.sub.z /.delta.x in the x-direction and G.sub.y =.delta.B.sub.z /.delta.y in the y-direction. Each of these saddle-shaped individual coils has straight conductor sections extending in the z-direction and arc-shaped conductor sections extending in the circumferential direction of the carrier body perpendicular to the z-axis. Current is conducted through the respective pairs of coils for each of the x and y directions such that the current flow directions are the same in adjacent straight conductor sections of the individual coils of each coil pair, but are opposed in the straight conductor sections of the coils which are arranged symmetrically with respect to the plane of symmetry. Such a gradient coil system is known from U.S. Pat. No. 3,569,823.
In the field of medical diagnostics, imaging methods have been proposed wherein an image similar to an X-ray tomogram is constructed by numerical or measurement analysis of integrated proton resonance signals from the spatial spin density and/or relaxation time distribution of a human body to be examined. The corresponding method is also known as "zeugmatography" or nuclear spin tomography. See: "Nature", volume 242, 1973, pages 190 to 191.
According to the known methods of nuclear spin tomography three different kinds of coil systems are required, in principle. One magnet is required to generate a stationary base field B.sub.z which must be as homogeneous as possible and having an order of magnitude of between 0.05 to 0.5 Tesla. Magnetic field B.sub.z is assumed to be oriented, for example, in the z-direction of an orthogonal x, y, z coordinate system. Moreover, the z-direction is the examination axis along which a body, particularly a human body to be examined, is placed in the magnetic field. The coordinate origin is to be situated in the imaging, or examination region. Furthermore, a high-frequency coil arrangement is to be provided for the corresponding precession frequency of the nuclear spin to be considered, in order to excite the nuclear spin, and optionally, to receive the induction signals. If the high-frequency coil arrangement is used for detecting these signals, a separate receiving coil system may also be provided. Finally, a system of gradient coils is needed which generate a preferably orthogonal set of supplementary fields G.sub.z =.delta.B.sub.z /.delta.z; G.sub.x =.delta.B.sub.z /.delta.x; and G.sub.y =.delta.B.sub.z /.delta.y. These supplementary fields are small in comparison with the base field B.sub.z which is oriented in the z-direction. Only the gradient fields which are switched on in the predetermined sequence permit a distinction in the location due to the shape of the precession frequency of the nuclei. See, for example, "Journal of Magnetic Resonance", volume 18, 1975, pages 69 to 83; volume 29, 1978, pages 355 to 373.
If the gradients G.sub.x, G.sub.y, and G.sub.z in an imaging region are not constant to a high degree, but are still functions of the location itself, blurred, distorted, and artifical images are generated. Linearity of the gradient fields and the constancy of their derivatives G.sub.x, G.sub.y, and G.sub.z in the imaging region are therefore an essential condition for high image quality of nuclear spin tomographic apparatus.
Generally, the three gradients can be generated by magnetic quadrupoles. The fact that the coils for generating the gradients must be arranged inside the base field magnet must be taken into consideration in the design of nuclear magnetic resonance apparatus. Thus, sufficient space must be left for placing the human body to be examined.
An analytic derivation of the geometry of such coil systems can be obtained from the above-mentioned U.S. Pat. No. 3,569,823. Thus, the coils in the coil system are to produce a magnetic field which is developed into spherical functions which are as pure as possible. It is assumed here that the field-generating conductors are arranged on the outside and/or inside cylindrical surfaces of a hollow cylindrical support body. In such an arrangement, disturbances of the main spherical functions which are generated by the finite length of the conductors and their locations are analytically minimized.
The hollow cylindrical support body with the corresponding gradient coils can be inserted into a field magnet having an axis which coincides with the axis of the base magnet and which points, for example, in the z-direction of an orthogonal x, y, z coordinate system. The z-gradient G.sub.z is generated by two ring coils through which current flows in opposite directions. In order to generate the x-gradient G.sub.x two saddle-shaped coil pairs are placed on the support body. For the y-gradient G.sub.y, a corresponding system of four saddle-shaped coils is provided which are arranged opposite to the x-gradient coils either on the outer or inner cylindrical surfaces of the cylindrical support body, shifted by 90.degree. in the circumferential direction. The two pairs of individual coils of each coil set are arranged symmetrically with respect to an x-y plane which is oriented perpendicularly to the cylinder axis and extends through the center of the imaging region.
In the calculation of this coil system it is stipulated that high linearity of the gradient field in the radial x-y plane is achieved. The x-y plane also represents the plane of symmetry through the imaging region. In order to take pictures of the entire body using nuclear magnetic resonance tomographic apparatus, linearity in this field is required not only in a two dimensional plane imaging region, but in a spherical volume having a radius of, for example, 20 cm because it is desirable to orient the imaging plane in space in any manner desired. This, however, requires that the gradient field be linear over the entire volume. The gradients are to be constant down to less than 5% in order to prevent substantial distortion of the image.
It is, therefore, an object of the present invention to develop a gradient coil system wherein a dimensionally extending imaging region having high linearity of the x and/or y gradients is obtained in a relatively simple and inexpensive manner.